The invention relates to rotary drill bits, typically drag bits, for use in drilling holes in subsurface formations. As used herein, "drilling" will include coring as well as the drilling of full bore holes. The bits are of the kind comprising a bit body having a shank at one end for connection to a drill string, an operating end face at the other end, a plurality of cutting elements mounted at the end face, and a passage in the bit body for supplying drilling fluid to the end face for cooling and/or cleaning the cutting elements. At least some of the cutting elements each comprise a preform cutting element having a superhard front cutting face. The invention is particularly, but not exclusively, applicable to drill bits of this kind in which the cutting elements comprise preforms having a thin facing layer of polycrystalline diamond bonded to a backing layer of tungsten carbide. Various methods may be used for mounting such preform cutting elements on the bit body but such methods, and the general construction of bits of the kind to which the invention relates, are well known and will not therefore be described in detail.
When drilling deep holes in subsurface formations, it often occurs that the drill passes through a comparatively soft formation and strikes a significantly harder formation. Also there may be hard occlusions within a generally soft formation. When a bit using preform cutters meets such a hard formation the cutting elements may be subjected to very rapid wear.
In order to overcome this problem it has been proposed to provide, immediately adjacent the rearward side of at least certain of the cutting elements, a body of material impregnated with natural diamond. For example, in the case where the bit body is a matrix material formed by a powder metallurgy process, it is known to mount each cutting element on a hard support which has been cast or bonded into the material of the bit body and in one such arrangement the hard support has been impregnated with diamond.
With such an arrangement, during normal operation of the drill bit the major portion of the cutting or abrading action of the bit is performed by the cutting elements in the normal manner. However, should a cutting element wear rapidly or fracture, so as to be rendered ineffective, for example by striking hard formation, the diamond-impregnated support on which the element is mounted takes over the abrading action of the cutting element thus permitting continued use of the drill bit. Provided the cutting element has not fractured or failed completely, it may resume some cutting or abrading action when the drill bit passes once more into softer formation.
A serious disadvantage of such an arrangement is that abrasion of the diamond-impregnated support against the formation generates a great deal of heat and the resultant high temperature to which the adjacent cutting element is subjected tends to cause rapid deterioration and failure of the cutting element and/or its attachment to the support. The present invention therefore sets out to provide arrangements in which this disadvantage is reduced or overcome.
In other bits, surface set natural diamonds are mounted in the bit body in trailing relation to the preform cutting elements. However, once such a surface set diamond is lost, e.g. due to wear of the surrounding area of the bit body, any advantage thereof is likewise lost.